Alternating electric current rectifier of the selenium type



Patented'Aug. 31, 1943 ALTERNATINGY Emcrmc CUB-BENT RECTL mm or me SELENIUM 'ryrn Leslie Ernest Thompson and Alexander Jenkins,

London, England,- assignors, by mesne assignments, to The Union Switch and Signal Company, Swissvale, Pa, a corporation of Pennsylvania No Drawing. Application February 12, 1943, Se-

rial No. 475,654. In'Great Britain September 2,

This invention relates to alternating current rectifiers of the selenium type comprising a base plate or support provided with a layer or coating of selenium which may contain other substances or compounds of selenium, a counter electrode :yingapplied to the surface of a the selenium The invention is an improvement in or modincation 01 the invention for which a patent was applied for by us in Great Britain on the 13th October, 1939, and numbered 534,043 and consisting in an improved form of counter-electrode composed mainly of tin, cadmium or an alloy of these metals with the addition oia relatively small quantity of thallium sprayed or otherwise deposited on the suriace of the selenium layer or coating.

According to the present invention a relatively small quantity of mercury is added to the tincadmium-thallium alloy.-'

The invention is of particular advantage in cases in'that an electric iorming process is employed consisting in applying to the rectifier in the reverse direction a voltage which is high relatively to the normal voltage at which the rectiller operates.

Usually this reverse voltage is o! the order of 20 volts and if this voltage is exceeded both the,

ZCIaims. '(Cl.1 75366) a 0.3% is added to the tin-cadmium-thallium alloy, the bottom bend voltage can be reduced and the forward characteristic of the rectifier improved. If a high proportion of mercury is added, however, for example 1% the rectifier tends to deform and the proportion above stated viz. 0.3% of mercury is a satisfactory compromise reducing the bottom bend voltage and still allowing satisfactory forming. i

The invention is evidently not limited to the particular proportions given above by way of example and variations in this respect may evidently be made without exceeding the scope of the invention.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. In an alternating current rectifier of the type comprising a layer of selenium disposed on a base plate and having applied to its outer surface a counter-electrode composedmainly of tin,

cadmium or an alloy of these metals with the addition of a relatively small quantity of thallium, the improvement which consists in adding to the coimter-electrode a small quantity of mercury 

